User Login message box when opening blender to choose a different startup file on the start
This question is too open-ended to be answered in detail, so I'll answer generally. Before we talk about how to create such a solution, there are some issues with what you are trying to do. 

First, there is no "security" offered by what you are doing, so there is no reason to ask for a password. You can simply let the user choose from a list of startup profiles. If you wish to provide some kind of security, you need to use your operating system's user accounts. You said you are using windows, so you would want to create a separate windows account for each user.

Second, it's strange to be asking for "username", since the startup file is only a set of presets for new projects. If the user saves his work, and loads it later, loading the file will overwrite the startup file information. Choosing a different startup file would more likely be used to choose different "configurations" of blender, for different types of tasks. 

Therefore, the only sensible way to use the UI you are suggesting is actually to present a list of "startup configurations" and let the user pick between them. The following will explain one way you might do that.

You will need to create some custom UI for your startup-selector using python. I don't believe you can modify the splash screen from python, so I would do this by making a custom page in the user-preferences UI. This custom page will contain your UI and the logic to load the proper startup file. (See scripts/startup/bl_ui/space_userpref.py)

Once you have your custom UI for picking a startup file and loading it, you can make a startup file which loads this when blender starts. Configure blender the way you want, with only one view on the screen, showing the user-pref space, with your custom space-chooser showing. Then do File->Save Startup File. Now this will show every time blender is launched.

That should give you a startup-file selector on launch.